What sorting algorithm used while overriding compare method of comparator interface?

Question

 Collections.sort(ar, new Comparator() {
                      @Override
                        public int compare(Intervals o1, Intervals o2) {            


        

Answer 1

The Comparator and Comparable interface don't do any sorting, so there is no sorting algorithm there. They just compare two Objects, something you need if you want to sort a list of those objects.

Answer 2

Collections.sort() uses a variation of Timsort.

From the javadocs:

The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techiques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.

Note that the Collections.sort() algorithms gets a "black box" comparator, and uses the value it yields for each compare - without caring what is going on behind the scenes of the comparator.